The invention relates to rare earth doped optical fibers for use as fiber lasers, fiber amplifiers and super-fluorescent sources. More particularly, the invention relates to a method for fabrication of a preform in which its core is doped with rare earth elements along with certain preferred codopants.
When an optical fiber in which its core is doped with rare earth ions is pumped with radiation of a certain wavelength, the rare earth ions absorb the pump energy and subsequently emit radiation with wavelengths different from that of the pump radiation. This property of the rare earth ion has been utilized in making fiber lasers and amplifiers. Laser transitions are achieved over a wide range of wavelengths by incorporating an appropriate choice of rare earth dopants into host glass composition. A variety of rare earth ions including Nd, Er, Yb, Ho, Pr, Eu, Ce, Dy, Sm, Tb, Gd, Pm and Tm have been shown to provide useful laser actions.
Numerous methods have been developed in the prior art to incorporate rare earth ions into silica glass. These methods include solution-doping (See, e.g., Townsend et al., xe2x80x9cSolution doping technique for fabrication of rare earth doped fibers,xe2x80x9d Electron. Lett., 23(7):329, 1987; Tanaka, xe2x80x9cSolution doping of a silica,xe2x80x9d U.S. Pat. No. 5,474,588; Tanaka, xe2x80x9cErbium-doped silica optical fiber preform,xe2x80x9d U.S. Pat. No. 5,526,459; Cognolato, xe2x80x9cMethod of fabricating optical fibers by solution doping, U.S. Pat. No. 5,047,076), sol-gel (See, e.g., DiGiovanni et al., xe2x80x9cA new optical fiber fabrication technique using sol-gel dip coating,xe2x80x9d Proceedings of OFC, vol. 4, paper WA2, 1991), aerosol (See, e.g., Morse et al., xe2x80x9cAerosol techniques for fiber core doping,xe2x80x9d Proceedings of OFC, vol. 4, paper WA3, 1991) and vapor phase techniques (See, e.g., Poole et al., xe2x80x9cFabrication of low loss optical fibers containing rare earth ions,xe2x80x9d Electron. Lett., 21(17):737, 1985; MacChesney et al., xe2x80x9cMulticonstituent optical fiber,xe2x80x9d U.S. Pat. No. 4,666,247; Tuminelli et al., xe2x80x9cFabrication of high concentration rare earth doped optical fibers using chelates,xe2x80x9d J. Lightwave Tech., 8(11):1680, 1990).
It is known that the incorporation of certain modifiers or codopants in the proximity of the rare earth ions in the core can alleviate some deleterious effects on the emission of radiation with a desired wavelength and enhance the highly efficient conversion of the pump radiation to radiation of desired wavelength. These codopants include but not restricted to Al, P, Ge and B.
Although the beneficial effects of the preferred codopants and their presence in a close proximity of the rare earth ion have been well known, no attempt has been made to deliver the codopants preferentially to the close proximity of the rare earth ions. In the conventional doping methods, the rare earth element and preferred codopants are uniformly incorporated into a preform and their relative locations with respect to each other are usually dependent on a natural diffusion process, which occurs in the condensed phase and is very slow.
In addition, although it is necessary for a rare earth element and a preferred codopant to have certain minimum concentrations to manifest their effective performance, methods to control independently a final concentration of each element in a preform to maximize their effects have not been available. For example, in prior art, P (or B) doping would be achieved by adding POCl3 (or BCl3) to the stream of SiCl4 used to form the soot layer. This puts severe limitations on the porosity and amount of soot that is generated and consequently affects the amount of rare earth and codopant that can be incorporated. When a P (or B) doped soot is deposited, the soot tends to sinter at low temperatures. Such sintering reduces the porosity of the soot and, hence, the soot""s ability to absorb the solution with the rare earth and codopant compounds. The temperature of the soot deposition can be reduced to increase the porosity. However, it was found that to achieve the desired amount of P the deposition temperature has to be reduced to an extent to which soot generation ceases. Thus, the methods of prior art cannot provide the means for incorporating rare earth elements and codopants at desired high levels, respectively.
The present invention provides a method to fabricate a rare earth-doped preform for an optical fiber.
According to one aspect of the invention, a porous silica soot layer is deposited on an inner surface of a silica-based substrate tube at a deposition temperature. The porous silica soot layer is immersed in an impregnation solution having at least one rare earth element and one codopant element to preferentially deliver the codopant element to the close proximity of the rare earth element on the porous silica soot layer. The porous silica soot layer is dried from the impregnation solution with a stream of chlorine and inert gas at an elevated temperature. The rare earth element and the codopant element is oxidized at an oxidation temperature to form a rare earth element oxide and a codopant element oxide. Finally, the porous silica soot layer is sintered into a glass layer.
The porous silica soot layer is deposited by a modified chemical vapor deposition process at a deposition temperature from about 1300xc2x0 C. to about 1800xc2x0 C. with a preferable deposition temperature at about 1600xc2x0 C. The rare earth element has an atomic number of 57 through 71 such as Yb, Nd, and Er. The rare earth element is used in the form of a water or alcohol soluble compound such as chloride or nitrate to prepare the impregnation solution. The rare earth chloride used in the preferred embodiments is YbCl3.6H2O, NdCl3.6H2O, or ErCl3.6H2O. The codopant element is used to promote the solubility of the rare earth element or to modify the phonon energy of the silica soot layer. The codopant element includes Al, P, Ge and B, preferably in the form of soluble compound such as chloride in the case of aluminum. The duration for the immersing step is set long enough so that the porous silica soot layer is saturated with the impregnation solution. A typical duration is at least 0.5 hour, preferably about 1 hour. In a preferred embodiment, the porous silica soot layer is dried from the impregnation solution first with a stream of inert gas and subsequently with a stream of chlorine and inert gas at an elevated temperature. The duration for the first drying step ranges from about 0.5 hour to about 1 hour. The temperature for the second drying step ranges from 600xc2x0 C. to 1200xc2x0 C., preferably 800-1000xc2x0 C. The oxidation temperature ranges from 600xc2x0 C. to 1000xc2x0 C.
In a preferred embodiment, a mixture of a second codopant precursor and oxygen is flown over the porous silica soot layer during said sintering step. The second codopant precursor is a chloride selected from the group consisting of POCl3, GeCl4 and BCl3. The sintering step is conducted at a temperature between about 1500xc2x0 C. and about 2000xc2x0 C., preferably at a temperature of about 1800xc2x0 C.
According to another aspect of the invention, a porous silica soot layer is deposited on an inner surface of a silica-based substrate tube at a deposition temperature. The porous silica soot layer is immersed in an impregnation solution having at least one rare earth element. The porous silica soot layer is dried from the impregnation solution with a stream of chlorine and inert gas at an elevated temperature. The rare earth element is oxidized at an oxidation temperature to form a rare earth element oxide. The porous silica soot layer is sintered with a mixture of a codopant precursor and oxygen wherein the codopant precursor becomes a codopant oxide which is preferentially delivered to the close proximity of the rare earth oxide on the porous silica soot layer.
The porous silica soot layer is deposited by a modified chemical vapor deposition process at the deposition temperature from about 1300xc2x0 C. to about 1800xc2x0 C., preferably at about 1600xc2x0 C. The rare earth element has an atomic number of 57 through 71 such as Yb, Nd, and Er. The rare earth element is used in the form of a soluble compound such as chloride or nitrate. The rare earth chloride used in the preferred embodiments is YbCl3.6H2O, NdCl3.6H2O, or ErCl3.6H2O. The duration for the immersing step is set long enough so that the porous silica soot layer is saturated with the impregnation solution. A typical duration is at least 0.5 hour, preferably about 1 hour. In a preferred embodiment, the porous silica soot layer is dried from the impregnation solution first with a stream of inert gas and then with a stream of chlorine and inert gas at an elevated temperature. The duration for the first drying step ranges from about 0.5 hour to about 1 hour. The temperature for the second drying step ranges from 600xc2x0 C. to 1200xc2x0 C., preferably 800-1000xc2x0 C. The oxidation temperature ranges from 600xc2x0 C. to 1000xc2x0 C. The codopant precursor is a chloride selected from the group consisting of POCl3, GeCl4 and BCl3. The codopant element is used to promote the solubility of the rare earth element or to modify the phonon energy of the silica soot layer. The sintering step is conducted at a temperature between about 1500xc2x0 C. and about 2000xc2x0 C., preferably at a temperature of about 1800xc2x0 C.
In a preferred embodiment, the impregnation solution further comprises a codopant element to further promote the solubility of the rare earth element. The codopant element includes Al, P, Ge and B, preferably in the form of water or alcohol soluble compounds such as chloride.